Determination of pore fluid pressure is an important aspect of subterranean drilling, exploration and completion operations. Determination of pore fluid pressure is important in maintaining proper fluid pressures to maximize the effectiveness of drilling, production or other operations. For example, the drilling fluid pressure applied by drilling fluid pumped downhole through a drillstring must be sufficient to control hydrostatic pressure in a wellbore to prevent blowouts and maintain optimum drilling rates.
Typically, the pore fluid pressure at a point in a formation has been calculated by considering a difference between total vertical and effective vertical stress at the point of interest. Conventionally, total vertical stress is estimated by vertical integration of density data. On the other hand, there are different approaches for estimation of effective vertical stresses.
Total vertical stress distribution in the Earth may be affected by many factors including surface topology and density heterogeneities. The effect of these factors on total vertical stresses decays with depth below the surface or below the heterogeneity. For example, total vertical stresses are significantly affected by topology close to the surface, but with increasing depth, they approach the stress distribution for a horizontal surface at average elevation.
Conventionally used vertical integration of density implicitly assumes that the gravitational load of an infinitesimal rock element is completely transferred to the element below it. As a result of this assumption, the influence of the gravitational load of an element on the vertical stress distribution does not decay with depth but is transferred to all the elements below it. Depending on the surface topography and density distribution, this assumption can result in overestimation or underestimation of the total vertical stresses and in turn, overestimation or underestimation of the formation pore pressures derived from the total vertical stresses.